What does important in css mean?

In css, important means "important" or "authoritative" and is used to increase the application priority of specified CSS style rules; it will be added to the end of the CSS style value to give the style More weight, syntax "selector{style:value!important;}". Using important rules has no negative impact on performance; however, from a maintainability perspective, unless absolutely necessary, you should avoid using important rules as much as possible, and it should only be used in special circumstances.

The operating environment of this tutorial: Windows 7 system, CSS3&&HTML5 version, Dell G3 computer.

In CSS, important means "important" or "authoritative".

important is a CSS rule used to increase the application priority of a specified CSS style rule; it is added to the end of a CSS value to give it more weight.

Syntax:

选择器{样式:值!important;}

important provides developers with a way to increase the weight of styles. It should be noted that !important is a declaration of the entire style, including the attributes and attribute values ​​of this style.

In CSS, style rules are applied to elements in a cascading fashion. The higher the weight in the following list, the smaller the weight:

• Browser style: It is the default style declared by the web browser.

• User-declared styles: are custom styles set by users using browser options or modified through developer debugging tools.
  

• Styles declared in development: are styles declared by website developers in CSS style sheets.
  

• Developer-declared styles with !important rules.
  

• User styles with !important rules.

The function of !important is to increase the priority of the specified style attribute.

Generally, when we set the style attribute for the same element, we set two or even more values ​​for the same attribute, like the following:

p { /* 例1 */
color:red;
color:yellow;
color:blue;
}

This is the subsequent statement The properties take effect and the font is displayed in blue.

Suppose we want me to write as above, but I want the final effective font to be displayed in red, then we can set red!important:

p { /* 例2 */
     color:red !important;    /* 设置了优先级 */
     color:yellow;
     color:blue;
}

Then the font will be displayed in red at this time.

If there are multiple style attributes set with !important, like the following:

p { /* 例3 */
     color:green !important;
     color:red !important;
     color:yellow !important;
     color:blue;
}

Then it will still be the same as when it is not set, and the last one declared will take effect. , that is to say, the final effective color of the above example is yellow.

##We know that the more specific the CSS selector is, the higher the priority, for example:

 
     <style> 
        p.p1 { color:blue; } 
        p { color:red; }
     </style>
 

    <p>p标签</p>

Finally, the

element is displayed in blue.

If we set !important to color:red;

 
    <style>
        p.p1 { color:blue; }
        p { color:red !important; }    /* 设置了优先级 */
    </style>



    <p>p标签</p>

The final

element will be displayed in red.

What if this

element has an inline style set?

    <style>
        p { color:red !important; }
    </style>


    <p>p标签</p>

Finally this

element is displayed in red. In other words, the style attribute declared !important will override any other declaration.

When to use !important rules?

You should not use !important rules unless absolutely necessary; using !important rules will break the natural cascading effect of the style sheet, making the code difficult to maintain. However, there are some situations where you have to use it! important:

1. When testing and debugging the website, the !important rule is very useful.

If we have some CSS issues in our code and want to make sure a specific style is applied, we can use the !important rule to temporarily fix the problem on the site until we find a better way (which may take some time).

2. Output style sheet

!important rules can also be used for output style sheets to ensure that styles are applied without being overwritten by anything else.

Conclusion

Using !important has no negative impact on performance; but from the perspective of maintainability, unless absolutely necessary, use !important should be avoided as much as possible rule, it should only be used in special circumstances.

!important in Internet Explorer

　　但在IE 6及更早的版本中，不支持我们这种写法。像例3，在IE5（我用的是IE11然后再控制台中切换到IE5的方式模拟的，IE11没有模拟IE6，凑合凑合_(:з」∠)_）中运行效果是这样的：

　　

　　没错，最终它还是显示为蓝色，!important被忽略了。让我们换一种写法：

p { color:blue; }
p { color:yellow; }
p { color:red !important; }/* 设置了优先级 */

　　这个时候就可以显示为红色了：

　　

　　再看看这个例子：

p { color:blue; }
p { 
   color:red !important;    /* 设置了优先级 */
   color:yellow;
}

　　在模拟IE5的浏览器中运行效果如下：

　　

个人理解：IE6及更早版本的IE浏览器中运行!important就好比说"大家都是针对这个元素设置的样式，color样式请优先使用我这个CSSRule对象（被{}包裹起来的一些样式属性）中的color样式"，而不是说"就使用我指定的color样式"。

（学习视频分享：web前端入门）

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